1. Field of the Invention
The invention relates to processes for producing an underground zone of fragmented and pervious material which may be used as a storage area, as a passage or aquifer, for treatment of materials, for ore body leaching or recovery of carbonaceous materials from subterranean deposits, etc. It has particular value in the extraction of oil and hydrocarbon gases from subterranean oil shale deposits by in situ retorting.
2. Description of Prior Art
As is well known, very large amounts of hydrocarbon oil are contained in subterranean oil shale deposits and the economic recovery of such oil and/or hydrocarbon gases has been a long-sought-after objective. Access to the oil shale may be obtained by removing the overburden by a strip mining procedure. However, such procedure is both costly and deleterious to the terrain. Efforts have, accordingly, been made to extract the oil and valuable hydrocarbon gases by the production of subterranean retorts in the oil shale deposits with minimum disturbance of the land surface. An example of such a procedure is disclosed in U.S. Pat. No. 4,037,657.
Heretofore, in the firing of charges emplaced in the subterranean ore body some lifting of the overburden has been accomplished to provide an in situ retort chamber. It is difficult, however, to effect such raising of the overburden without the production of substantial cracks which need to be sealed in order to provide an effective lid for the retort chamber. Another and effective technique is to sever a block of overburden by defining planes of discontinuity peripherally around the block. Such planes may be present by reason of local jointing or they may be produced by explosive fracturing, i.e., pre-splitting. An initiating round of explosive charges may then be used to raise the block, and this may be followed by the firing of adjacent charges in the deposit to effect fragmentation and further raising of the overburden.